The present invention relates to control techniques for a pickup of an optical-type video disc player, etc., and more particularly to control information detecting method and apparatus in a servo system performing focus and tracking controls for the pickup, for example.
Since the present invention is preferably applicable to an optical-type video disc player, the subsequent explanation will be made in connection with a servo system for a pickup of the optical-type video disc player though any limitation to the present invention is not intended thereby. Such a servo system requires an automatic focus control for focussing the spot of read-out light onto a recording surface of a generally disc-shaped recording medium having information recorded thereon and a so-called tracking control for accurately positioning the light spot on a recording track. Various prior art measures have been proposed for such controls and for the detection of control signals used therein. A known signal detecting method for the automatic focus control includes a scheme in which light passing out of the optical axis of a lens is used together with a two-separate photodetector, a scheme in which a cylindrical lens and a four-separate photodetector are used, and an wobbling scheme in which the position of a lens, etc. is slightly moved or wobbled. On the other hand, a known signal detecting method for the tracking control includes a scheme in which two light spots and two individual photodetectors are used, and an wobbling scheme in which a light beam is slightly moved or wobbled in a lateral or transversal direction in relation to the optical axis. Some of such schemes are disclosed in four articles published by the journal of the SMPTE, Vol. 83, (July 1974): the Compaan et al article entitled "The Philips `VLP` system" in pp. 564-566, the Bussche et al article entitled "Signal processing in the Philips `VLP` system" in pp. 567-571, the Janseen et al article entitled "Control mechanisms in the Philips `VLP` record player" in pp. 576-579, and the Bouwhuis et al article entitled "The optical scanning system of the Philips `VLP` record player" in pp. 572-575.
The above-described either scheme, other than the wobbling scheme, requiring additional photodetector or optical system as a control means in addition to a read-out means for reading out the recorded information, involves an inherent problem that a special detector or a plurality of optical systems must be employed, thereby resulting in a complicated device structure, an expensive device cost and difficulties in adjustment accompanying poor reliability.
To the contrary, the wobbling scheme is advantageous in that no additional or extra photodetector is required, thereby making the optical system simple. However, it has hitherto been difficult to simultaneously employ the wobbling scheme to both the automatic focus control and the tracking control. The reason is that if small movements or wobbles to be used in the respective control systems for automatic-focus and tracking are applied to a light beam to obtain signals for the automatic-focus control and the tracking control, the light input onto the detector would involve variations in a form of the product of the respective small wobbles, thereby making it difficult to separately and individually obtain signals for the respective controls.